This invention relates to (a) processes for removing selenocyanate, arsenic, iron, and vanadium from an aqueous fluid and, in particular, to a method for reducing the concentration of these materials in an oil refinery effluent, and (b) an apparatus for removing selenate and selenite from an aqueous fluid.
Several parts of the San Francisco Bay Estuary, including Suisun Bay, San Pablo Bay, and the Carduinez Straits, have been classified by the U.S. Environmental Protection Agency (EPA) under Section 304(1) of the Clean Water Act as impaired due to the impact of selenium on waterfowl. As a result of this classification, in 1991 the San Francisco Regional Water Quality Control Board (Board) amended the National Pollution Discharge Elimination System (NPDES) permits for the six refineries discharging into the estuary to include specific permit limits on total selenium, with a compliance deadline of Dec. 12, 1993. Each refinery was given a mass discharge limit for total selenium. This limit was based on either previous performance, or an effluent concentration of 50 parts per billion (ppb), whichever resulted in the lower limit.
Even before the establishment of the limit on total selenium by the Board, the various refineries had been working to develop control strategies, including treatment options, to meet the 50 ppb limit on total selenium. Nevertheless, despite studying over 50 treatment technology variations, no treatment option was found or developed to achieve this limit which did not have serious negative environmental consequences, such as generation of large amounts of hazardous sludge or increased levels of copper in the effluent.
Among the numerous technologies tested was an ion exchange process described in U.S. Pat. No. 4,915,928 and several acidification techniques. However, since no new control option could be developed in time to meet the December, 1993 compliance deadline for the 50 ppb limit, the refineries requested an extension of the compliance period in a proposal to the San Francisco Regional Water Quality Control Board entitled Removal of Selenium from Refinery Discharges ("Extension Proposal"). In the Extension Proposal, the ion exchange process is summarized as follows:
Chevron Ion Exchange Process
Chevron developed an ion exchange process using a Type 1 gel resin. The process concentrates selenium from dilute solutions and produces a stable selenium product. The process was tested at the pilot scale and achieved greater than 90% removal for two SSW {stripped sour water} streams, but only poor removal for two other SSW streams which contained the majority of selenium {citing Chevron. Control strategy for selenium. Mar. 31, 1989. Report submitted to Regional Water Quality Control Board ("Chevron Report")}. Even with the stream on which the ion exchange was effective, the process was vulnerable to upset conditions in the sour water strippers {citing Chevron Report}.
Furthermore, the Extension Proposal (Table 3, page 11) notes that the Chevron ion exchange technology is "not feasible despite observed effectiveness." In addition, the Chevron Report at page 21 states:
The Chevron Ion Exchange Process for removing selenium from Richmond Refinery stripped sour waters is not practicable because:
* Costs, roughly $11,000/lb removed, are orders of magnitude above what EPA reports in its cost-effectiveness guidance; PA1 * Receiving water selenium concentrations are orders of magnitude below the level the SWRCB {sic; State Regional Water Quality Control Board} recommends as below harmful effects. PA1 Acidification/Degassing PA1 Selenium is present in sour water as the selenide (Se.sup.2-) or biselenide (HSe.sup.-) anion as would be expected if selenium behaves similarly to sulfur, stripping of the sample at reduced pH should remove selenium. Exxon tested this method on samples of (unstripped) sour water and found that a significant fraction of the selenium could be removed by heating samples of sour water at low pH {citation omitted}. This is consistent with their finding that selenium is stripped along with sulfur in the sour water strippers. However, Exxon also found that a maximum of about 60% selenium removal could be achieved at a pH of 1. Such a low pH is impractical to operate at, and 60% removal from sour water would not be nearly sufficient to meet a 50 ppb limit. Acidification/degassing on sour water stripper bottoms (stripped sour water--SSW) was tested by Exxon, Shell, and Unocal {citations omitted}. Only poor removal (&lt;30%) could be obtained when either air or nitrogen sparging at pH &lt;2 was used. Acidification followed by aeration may also remove soluble selenium by oxidizing selenide to elemental selenium which is then allowed to precipitate. Chevron found that a maximum of 47% removal could be achieved in this manner, and the results were not very reproducible.
The Extension Proposal comments on the acidification procedures as follows: